A common method of converting a small current signal into output voltage signal utilizes an operational amplifier (op-amp) and a resistive feedback element to convert from input current to output voltage. An implementation of this method is described in chapter four, pages 175–261, of The Art of Electronics 2nd edition, Horowitz, P. and Hill, W., Cambridge University Press (1989). Resistive feedback op-amp circuits suffer from a basic limitation. Specifically, the measurement of picoampere current levels with high bandwidth is limited by the noise of the preamplifier system converting the small input current into a larger signal for subsequent signal processing. In applications using resistive feedback, the limiting noise is the thermal noise associated with the feedback resistor.
It may be possible to attempt to lower the feedback resistor value to reduce the resistor noise contribution and increase the bandwidth of the output. The noise of the feedback resistor scales with the square root of the resistor size. The transresistance scales with the resistor size. Accordingly, the transresistance through the preamplifier circuitry may not be sufficiently large to dominate the noise of the subsequent signal processing stages.
U.S. Pat. No. 6,380,790, which is incorporated herein by reference, discloses a circuit suitable for measuring small current levels that utilizes an integrator topology instead of an op-amp with a feedback resistor. The '790 patent utilizes capacitive feedback and, hence, requires periodic resets if the input current has a net direct current (DC) component. The reset operations require the use of switches to discharge the integrating capacitors. The switching operations cause transient injection of current.
The dominant noise limitation for circuits implemented according to the '790 topology is:Inoise=K*Cin*Enop—amp*BW1.5, whereInoise is the effective input noise current for the preamplifier, Cin is the input capacitance of the preamplifier and the transducer, Enop—amp is the thermal noise of the op-amp, BW is the desired signal bandwidth, and K is an appropriate proportionality constant.
Given the preceding noise limitation, known implementations of circuits according to the '790 topology may only measure picoampere current levels with a bandwidth on the order of 10 kHz. Specifically, the internal capacitance due to the switching elements and the general wiring capacitance prevents measurement of picoampere current levels having a bandwidth of 100 kHz or greater.